The present invention relates to an apparatus for the calcination of materials and uses therefor.
A variety of industrial processes involve the use of calcination to thermally decompose materials either to aid in the purification of materials or for use in an industrial process. Generally, calcination processes involve exposing the materials to be calcined to heat to thermally decompose the materials. Thus, calcination differs from thermal drying of materials in which free water is driven off by exposure to increased temperatures. In contrast, calcination involves changing the chemical composition of the material.
A number of apparatus are known for calcination processes. For example, rotary direct-fired calciners use an open flame as a heat source and therefore, necessitate the use of combustion air. Also, vertical fluid bed calciners use heated gas in direct contact with the material to be calcined.
Despite the well-known use of calcination, a number of problems exist in the use of conventional calcination processes. For example, the emission of by-products such as particulates causes pollution concerns. Additionally, a number of calcination processes are not energy efficient because much of the energy from the process is released to the atmosphere in the form of heat.
Further, many calcination processes which operate at high temperatures, such as use of open flame calciners, unevenly heat the material to be calcined. For example, in open flame rotary calciners, material contacting the flame may experience a temperature close to 1000xc2x0 C., even though the average temperature in the calciner may be significantly below that temperature. In this manner, some particles may not be fully calcined and some may be combusted. Alternatively, some larger particles may be calcined on the outside, but not on the inside of the particle. This type of disadvantage can also have significant negative effects on downstream processing because the material exiting the calcination process is not uniform in its chemical composition. Therefore, subsequent processing will have more variable results than if the material from the calcination process was uniform in nature.
As a result of the above disadvantages of known calcination technology, there remains a need for improved calcination apparatus and methods of use.
One embodiment of the present invention is an indirect heat calcination apparatus for calcining materials. The apparatus includes a feed inlet, a calcining chamber which is interconnected to the feed inlet, an indirect heating element within the calcining chamber to transfer heat from a heated fluid to the material, a bed plate located below the indirect heating element within the calcining chamber, and a product collection chute which is connected to the calcining chamber. The apparatus can also include a plurality of holes on the bed plate and a gas inlet for introducing a fluidizing gas into the apparatus through the bed plate holes. The apparatus can include an exhaust port located above the calcining chamber. The exhaust port can also include an expansion chamber for slowing the velocity of gas exiting the calcining apparatus. The indirect heating element of the apparatus can be, for example, coils within the calcining chamber which conduct the heated fluid through the chamber. Thus, the indirect heating element can include a fluid inlet port and a fluid outlet port. The apparatus can also include a plurality of calcining zones which are defined by compartmental walls.
The present invention includes a calcining process for treating a saline mineral which includes introducing the saline mineral to a calcining chamber, heating the saline mineral to a temperature of less than about 350xc2x0 by contacting it with an indirect heating element and removing the calcined material from the chamber. In this embodiment, the calcining chamber can include a bed plate located below the indirect heating element and having a plurality of bed plate holes and a gas inlet for introducing a fluidizing gas into the chamber through the bed plate holes. The calcining apparatus can also include an exhaust port located above the calcining chamber which can have an expansion chamber for slowing the velocity of exiting gas. The apparatus can also include a plurality of calcining zones defined by compartmental walls.
Other processes of the present invention include processes for calcining material and subsequent processing of the material. For example, such processing can include purification, such as crystallization.
An additional process of the present invention is a method for reducing the emission of pollutants during calcining. This process includes heating a saline mineral in a calcining vessel wherein the calcination step produces a gas comprising water vapor and a pollutant. The calcining gas is removed from the calcining vessel to an outlet and at least a portion of the water vapor in the calcining gas is condensed.
In this manner, a portion of the pollutants in the calcining gas are removed. This process can also include the use of a heat source for calcining which is not in direct fluid communication with the material to be calcined. In further a aspect, the material is calcined at temperatures less than about 250xc2x0 C.